1. Field of the Invention
The present invention relates to a broadband wireless access (BWA) communication system, and more particularly to a system and a method for performing a soft handover.
2. Description of the Related Art
In the 4th generation (4G) communication system, which is the next generation communication system, extensive research is being conducted to provide users with services having various Qualities-of-Service (QoSs) at high speeds. In the current 4G communication system, extensive research is being conducted to develop a new type of communication system ensuring the mobility of a mobile station (MS) and a QoS in a BWA communication system such as a wireless Local Area Network (LAN) system and a wireless Metropolitan Area Network (MAN) system capable of supporting relatively high transmission speeds. Representatives of such communication systems are the Institute of Electrical and Electronics Engineers (IEEE) 802.16a communication system and the IEEE 802.16e communication system.
The IEEE 802.16a/d-based communication system and the IEEE 802.16e-based communication system use an orthogonal frequency division multiplexing (OFDM)/orthogonal frequency division multiple access (OFDMA) scheme for a physical channel of the WMAN system in order to support a broadband transmission network. The IEEE 812.16a communication system is a single structure system that does not reflect the mobility of a Subscriber Station (SS) at all, in which the SS is in a stationary state. In contrast, the IEEE 812.16e-based communication system is a system realized by supplementing the IEEE 812.16a-based communication system with the ability to handle the mobility of an SS. Hereinafter, a mobile SS will be referred to as a MS. Accordingly, in the IEEE 812.16e-based communication system, the study of handover according to the mobility of the MS is being activated.
Handover refers to an operation enabling smooth provision of services to an MS which is moving between base stations (BSs). Such handover may have two schemes (a hard handover scheme and a soft handover scheme). The hard handover scheme and the soft handover scheme will be described.
The hard handover scheme denotes a scheme through which, when an MS moves between base stations, the MS releases a channel setup with an anchor base station (a serving base station currently providing a service to the MS) and sets up a channel with an active non-anchor base station (a target base station) in the shortest period of time, so that the service may be continuously provided. The serving base and the target base station will be referred to as an anchor base station and an active non-anchor base station, respectively, for the purpose of description. The hard handover causes a ping-pong phenomenon of continuously alternating channel release and setup when channel states are continuously inferior, for example, when the MS moves across a cell boundary region.
The soft handover scheme denotes a scheme through which, when an MS moves between base stations, the MS has channels set up with both an anchor base station and an active non-anchor base station, is relocated in the non-anchor base station while simultaneously receiving signals from both the anchor base station and the non-anchor base station through the setup channel, and then releases the channel from the anchor base station, so that the service is continuously provided. Accordingly, when the MS performs a handover operation through the soft handover scheme, the MS receives downlink signals from a plurality of base stations (i.e. the anchor base station and the non-anchor base station) and improves quality of the received downlink signals by combining the downlink signals received from the base stations.
The signal combining scheme may be classified into a soft combining scheme and a selection diversity scheme.
The soft combining scheme is a scheme of demodulating each of the signals received from a plurality of base stations and combining the signals in a front end of a channel decoder. The selection diversity scheme is a scheme of de-modulating and channel-decoding each of the signals received from a plurality of base stations and then selecting a signal with the best quality from among the channel-decoded signals.
The MS performing a handover operation through the soft handover scheme may obtain a soft combining gain by simultaneously receiving signals through channels set up with both the anchor base station and the non-anchor base station. In order to obtain the combining gain, the MS must receive information regarding channels (channel allocation information) set up with both the anchor base station and the non-anchor base station and receive signals from the anchor base station and the non-anchor base station using the channel allocation information.
However, in the typical broadband wireless access system (e.g., the IEEE 812.16e-based communication system), it is difficult for the MS to receive channel allocation information of an active non-anchor base station because an anchor base station reports only information regarding a channel allocated by the anchor base station.